inet

Access to TCP/IP Protocols

Provides access to TCP/IP protocols.

See also ERTS User's Guide, Inet configuration for more
information on how to configure an Erlang runtime system for IP
communication.

Two Kernel configuration parameters affect the behaviour of all
sockets opened on an Erlang node:
inet_default_connect_options can contain a list of default
options used for all sockets returned when doing connect,
and inet_default_listen_options can contain a list of
default options used when issuing a listen call. When
accept is issued, the values of the listensocket options
are inherited, why no such application variable is needed for
accept.

Using the Kernel configuration parameters mentioned above, one
can set default options for all TCP sockets on a node. This should
be used with care, but options like {delay_send,true}
might be specified in this way. An example of starting an Erlang
node with all sockets using delayed send could look like this:

Note that the default option {active, true} currently
cannot be changed, for internal reasons.

Addresses as inputs to functions can be either a string or a
tuple. For instance, the IP address 150.236.20.73 can be passed to
gethostbyaddr/1 either as the string "150.236.20.73"
or as the tuple {150, 236, 20, 73}.

Functions

close/1

Closes a socket of any type.

get_rc/0

Returns the state of the Inet configuration database in
form of a list of recorded configuration parameters. (See the
ERTS User's Guide, Inet configuration, for more information).
Only parameters with other than default values are returned.

format_error/1

Returns a diagnostic error string. See the section below
for possible Posix values and the corresponding
strings.

getaddr/2

Returns the IP-address for Host as a tuple of
integers. Host can be an IP-address, a single hostname
or a fully qualified hostname.

getaddrs/2

Returns a list of all IP-addresses for Host.
Host can be an IP-address, a single hostname or a fully
qualified hostname.

gethostbyaddr/1

Returns a hostent record given an address.

gethostbyname/1

Returns a hostent record given a hostname.

gethostbyname/2

Returns a hostent record given a hostname, restricted
to the given address family.

gethostname/0

Returns the local hostname. Will never fail.

getifaddrs/0

Returns a list of 2-tuples containing interface names and the
interface's addresses. Ifname is a Unicode string.
Hwaddr is hardware dependent, e.g on Ethernet interfaces
it is the 6-byte Ethernet address (MAC address (EUI-48 address)).

The {addr,Addr}, {netmask,_} and {broadaddr,_}
tuples are repeated in the result list iff the interface has multiple
addresses. If you come across an interface that has
multiple {flag,_} or {hwaddr,_} tuples you have
a really strange interface or possibly a bug in this function.
The {flag,_} tuple is mandatory, all other optional.

Do not rely too much on the order of Flag atoms or
Ifopt tuples. There are some rules, though:

Immediately after {addr,_} follows {netmask,_}

Immediately thereafter follows {broadaddr,_} if
the broadcast flag is not set and the
pointtopoint flag is set.

Any {netmask,_}, {broadaddr,_} or
{dstaddr,_} tuples that follow an {addr,_}
tuple concerns that address.

The {hwaddr,_} tuple is not returned on Solaris since the
hardware address historically belongs to the link layer and only
the superuser can read such addresses.

On Windows, the data is fetched from quite different OS API
functions, so the Netmask and Broadaddr
values may be calculated, just as some Flag values.
You have been warned. Report flagrant bugs.

getopts(Socket, Options) -> {ok, OptionValues} | {error, posix()}

Socket = term()

Options = [Opt | RawOptReq]

Opt = atom()

RawOptReq = {raw, Protocol, OptionNum, ValueSpec}

Protocol = integer()

OptionNum = integer()

ValueSpec = ValueSize | ValueBin

ValueSize = integer()

ValueBin = binary()

OptionValues = [{Opt, Val} | {raw, Protocol, OptionNum, ValueBin}]

Gets one or more options for a socket.
See setopts/2
for a list of available options.

The number of elements in the returned OptionValues
list does not necessarily correspond to the number of options
asked for. If the operating system fails to support an option,
it is simply left out in the returned list. An error tuple is only
returned when getting options for the socket is impossible
(i.e. the socket is closed or the buffer size in a raw request
is too large). This behavior is kept for backward
compatibility reasons.

A RawOptReq can be used to get information about
socket options not (explicitly) supported by the emulator. The
use of raw socket options makes the code non portable, but
allows the Erlang programmer to take advantage of unusual features
present on the current platform.

The RawOptReq consists of the tag raw followed
by the protocol level, the option number and either a binary
or the size, in bytes, of the
buffer in which the option value is to be stored. A binary
should be used when the underlying getsockopt requires
input
in the argument field, in which case the size of the binary
should correspond to the required buffer
size of the return value. The supplied values in a RawOptReq
correspond to the second, third and fourth/fifth parameters to the
getsockopt call in the C socket API. The value stored
in the buffer is returned as a binary ValueBin
where all values are coded in the native endianess.

Asking for and inspecting raw socket options require low
level information about the current operating system and TCP
stack.

As an example, consider a Linux machine where the
TCP_INFO option could be used to collect TCP statistics
for a socket. Lets say we're interested in the
tcpi_sacked field of the struct tcp_info
filled in when asking for TCP_INFO. To
be able to access this information, we need to know both the
numeric value of the protocol level IPPROTO_TCP, the
numeric value of the option TCP_INFO, the size of the
struct tcp_info and the size and offset of
the specific field. By inspecting the headers or writing a small C
program, we found IPPROTO_TCP to be 6,
TCP_INFO to be 11, the structure size to be 92 (bytes),
the offset of tcpi_sacked to be 28 bytes and the actual
value to be a 32 bit integer. We could use the following
code to retrieve the value:

setopts(Socket, Options) -> ok | {error, posix()}

Sets one or more options for a socket. The following options
are available:

{active, true | false | once}

If the value is true, which is the default,
everything received from the socket will be sent as
messages to the receiving process. If the value is
false (passive mode), the process must explicitly
receive incoming data by calling gen_tcp:recv/2,3
or gen_udp:recv/2,3 (depending on the type of
socket).

If the value is once ({active, once}),
one data message from the socket will be sent
to the process. To receive one more message,
setopts/2 must be called again with the
{active, once} option.

When using {active, once}, the socket changes
behaviour automatically when data is received. This can
sometimes be confusing in combination with connection
oriented sockets (i.e. gen_tcp) as a socket with
{active, false} behaviour reports closing
differently than a socket with {active, true}
behaviour. To make programming easier, a socket where
the peer closed and this was detected while in
{active, false} mode, will still generate the
message
{tcp_closed,Socket} when set to {active, once} or {active, true} mode. It is therefore
safe to assume that the message
{tcp_closed,Socket}, possibly followed by socket
port termination (depending on the exit_on_close
option) will eventually appear when a socket changes
back and forth between {active, true} and
{active, false} mode. However,
when peer closing is detected is all up to the
underlying TCP/IP stack and protocol.

Note that {active,true} mode provides no flow
control; a fast sender could easily overflow the
receiver with incoming messages. Use active mode only if
your high-level protocol provides its own flow control
(for instance, acknowledging received messages) or the
amount of data exchanged is small. {active,false}
mode or use of the {active, once} mode provides
flow control; the other side will not be able send
faster than the receiver can read.

{broadcast, Boolean}(UDP sockets)

Enable/disable permission to send broadcasts.

{delay_send, Boolean}

Normally, when an Erlang process sends to a socket,
the driver will try to immediately send the data. If that
fails, the driver will use any means available to queue
up the message to be sent whenever the operating system
says it can handle it. Setting {delay_send, true}
will make all messages queue up. This makes
the messages actually sent onto the network be larger but
fewer. The option actually affects the scheduling of send
requests versus Erlang processes instead of changing any
real property of the socket. Needless to say it is an
implementation specific option. Default is false.

{dontroute, Boolean}

Enable/disable routing bypass for outgoing messages.

{exit_on_close, Boolean}

By default this option is set to true.

The only reason to set it to false is if you want
to continue sending data to the socket after a close has
been detected, for instance if the peer has used
gen_tcp:shutdown/2
to shutdown the write side.

{header, Size}

This option is only meaningful if the binary
option was specified when the socket was created. If
the header option is specified, the first
Size number bytes of data received from the socket
will be elements of a list, and the rest of the data will
be a binary given as the tail of the same list. If for
example Size == 2, the data received will match
[Byte1,Byte2|Binary].

{keepalive, Boolean}(TCP/IP sockets)

Enables/disables periodic transmission on a connected
socket, when no other data is being exchanged. If
the other end does not respond, the connection is
considered broken and an error message will be sent to
the controlling process. Default disabled.

{nodelay, Boolean}(TCP/IP sockets)

If Boolean == true, the TCP_NODELAY option
is turned on for the socket, which means that even small
amounts of data will be sent immediately.

{packet, PacketType}(TCP/IP sockets)

Defines the type of packets to use for a socket.
The following values are valid:

raw | 0

No packaging is done.

1 | 2 | 4

Packets consist of a header specifying the number of
bytes in the packet, followed by that number of bytes.
The length of header can be one, two, or four bytes;
containing an unsigned integer in big-endian byte order.
Each send operation will generate the header, and the header
will be stripped off on each receive operation.

In current implementation the 4-byte header is limited to 2Gb.

asn1 | cdr | sunrm | fcgi | tpkt | line

These packet types only have effect on receiving.
When sending a packet, it is the responsibility of
the application to supply a correct header. On
receiving, however, there will be one message sent to
the controlling process for each complete packet
received, and, similarly, each call to
gen_tcp:recv/2,3 returns one complete packet.
The header is not stripped off.

The Hypertext Transfer Protocol. The packets
are returned with the format according to HttpPacket
described in
erlang:decode_packet/3. A socket in passive
mode will return {ok, HttpPacket} from gen_tcp:recv
while an active socket will send messages like {http,
Socket, HttpPacket}.

Note that the packet type httph is not
needed when reading from a socket.

{packet_size, Integer}(TCP/IP sockets)

Sets the max allowed length of the packet body. If
the packet header indicates that the length of the packet
is longer than the max allowed length, the packet is
considered invalid. The same happens if the packet header
is too big for the socket receive buffer.

{read_packets, Integer}(UDP sockets)

Sets the max number of UDP packets to read without
intervention from the socket when data is available.
When this many packets have been read and delivered
to the destination process, new packets are not read
until a new notification of available data has arrived.
The default is 5, and if this parameter is set too
high the system can become unresponsive due to
UDP packet flooding.

{recbuf, Integer}

Gives the size of the receive buffer to use for
the socket.

{reuseaddr, Boolean}

Allows or disallows local reuse of port numbers. By
default, reuse is disallowed.

{send_timeout, Integer}

Only allowed for connection oriented sockets.

Specifies a longest time to wait for a send operation to
be accepted by the underlying TCP stack. When the limit is
exceeded, the send operation will return
{error,timeout}. How much of a packet that actually
got sent is unknown, why the socket should be closed
whenever a timeout has occurred (see send_timeout_close).
Default is infinity.

{send_timeout_close, Boolean}

Only allowed for connection oriented sockets.

Used together with send_timeout to specify whether
the socket will be automatically closed when the send operation
returns {error,timeout}. The recommended setting is
true which will automatically close the socket.
Default is false due to backward compatibility.

{sndbuf, Integer}

Gives the size of the send buffer to use for the socket.

{priority, Integer}

Sets the SO_PRIORITY socket level option on platforms where
this is implemented. The behaviour and allowed range varies on
different systems. The option is ignored on platforms where the
option is not implemented. Use with caution.

{tos, Integer}

Sets IP_TOS IP level options on platforms where this is
implemented. The behaviour and allowed range varies on different
systems. The option is ignored on platforms where the option is
not implemented. Use with caution.

In addition to the options mentioned above, raw
option specifications can be used. The raw options are
specified as a tuple of arity four, beginning with the tag
raw, followed by the protocol level, the option number
and the actual option value specified as a binary. This
corresponds to the second, third and fourth argument to the
setsockopt call in the C socket API. The option value
needs to be coded in the native endianess of the platform and,
if a structure is required, needs to follow the struct
alignment conventions on the specific platform.

Using raw socket options require detailed knowledge about
the current operating system and TCP stack.

As an example of the usage of raw options, consider a Linux
system where you want to set the TCP_LINGER2 option on
the IPPROTO_TCP protocol level in the stack. You know
that on this particular system it defaults to 60 (seconds),
but you would like to lower it to 30 for a particular
socket. The TCP_LINGER2 option is not explicitly
supported by inet, but you know that the protocol level
translates to the number 6, the option number to the number 8
and the value is to be given as a 32 bit integer. You can use
this line of code to set the option for the socket named
Sock:

inet:setopts(Sock,[{raw,6,8,<<30:32/native>>}]),

As many options are silently discarded by the stack if they
are given out of range, it could be a good idea to check that
a raw option really got accepted. This code places the value
in the variable TcpLinger2: